Hi Throba,
I just had this problem on a computer I remote accessed that was painfully slow to render. Amongst other issues, they had a half gig file they were using as the source for a two second take off shot in every template.
Unfortunately there is no real way of doing what you request. My only suggestion is to create trimmed versions of all your stock footage to reduce archive size. The lower third files look like they can be combined into layered images.
It's actually Vegas doing the archiving. I tell it to archive with all required media, which includes everything it needs to open the project, source media, stock footage and music files. It does mean your archives will have huge numbers of duplicated files. If I didn't archive media, and someone deleted the original stock footage, none of the customer projects would open. I'm pretty sure Vegas updates the links too, so if you delete the stock footage manually, the customer's project will not open because it's lost the link to the original files and has no idea where to find them.
The only way to do this, would be to archive the source media and not the completed templates, which would mean you would need to recreate the customer's video if needed. If the templates had been changed, the customer would receive the new template rather than their original video. In this configuration, the open button on the Search tab would need to be disabled and you would have to use the Start over button. I'll add this option to the feature request list. "Only archive customer source media and photos. Do not archive completed projects or music."
Mick